The present invention relates to a directional coupler and a wireless communication device, and particularly, to technologies for improving the isolation characteristic and directivity of the directional coupler to reduce an insertion loss by bringing innovations into layouts of conductor patterns and the like within a laminate which forms part of the directional coupler.
A directional coupler (hereinafter sometimes referred to as the “coupler”) for extracting part of electric power propagated through a transmission line is an indispensable part for constructing a transmission circuit for a variety of wireless communication devices such as a mobile phone, a wireless LAN communication device, a communication device conforming to the Bluetooth (registered trademark) standard, and the like.
Specifically, a coupler forms part of adjusting means for controlling a transmission signal to keep its level constant, and this adjusting means comprises a power amplifier (hereinafter sometimes referred to as the “PA”) capable of control a gain, a coupler for detecting the level of a transmission signal, and an automatic power control circuit (hereinafter sometimes referred to as the “APC circuit”). An input transmission signal is amplified by the PA, and then output through the coupler. The coupler supplies the APC circuit with a monitor signal which has a level corresponding to the level of the transmission signal output from the PA. The APC circuit controls the gain of the PA such that the output of the PA remains constant in accordance with the level of the monitor signal (i.e., the level of the transmission signal). The transmission output is stabilized by such feedback control of the PA.
The foregoing coupler comprises a main line and sub-line which are disposed in close proximity to each other to make electromagnetic coupling therebetween. The main line for transmitting a transmission signal comprises an input terminal at one end thereof, and an output terminal at the other end thereof, respectively. The sub-line for detecting the level of a transmission signal comprises a coupling terminal at one end thereof and an isolation terminal at the other end thereof, respectively. Then, part of a transmission signal transmitted through the main line is extracted by the sub-line, and is outputted to the APC circuit through the coupling terminal, as a monitor signal.
Such couplers are also provided in general as a one-chip component which comprises a main line, a sub-line, a variety of terminals, and the like arranged within a laminate which includes a plurality of conductor layers with an insulating layer interposed between the respective conductor layers.
Main characteristics of a coupler may include insertion loss, degree of coupling, isolation, and directivity. The insertion loss is a loss caused by the coupler, and is desired to be lower. The degree of coupling indicates the ratio of power propagating in a forward direction (direction from an input terminal to an output terminal of a main line) to power extracted to a coupling terminal. The isolation indicates a leakage of power propagating in a reverse direction (direction from the output terminal to the input terminal of the main line) to the coupling terminal, and is desired to be higher (a smaller leakage). The directivity, in turn, indicates the difference between isolation and degree of coupling, and a better coupler is defined to exhibit a higher directivity (larger absolute value), allowing for the formation of a satisfactory APC circuit with few detection errors.
Documents related to such a coupler include JP-A-2002-280810 (Patent Document 1), JP-B2-3651401 (Patent Document 2), JP-B2-5472717 (Patent Document 3), JP-B2-5472718 (Patent Document 4), and JP-B2-4604431 (Patent Document 5).